Globular clusters in the outer galactic halo: New Hubble Space Telescope/advanced camera for surveys imaging of six globular clusters and the galactic globular cluster age-metallicity relation

Color-magnitude diagrams (CMDs) derived from Hubble Space Telescope (HST) Advanced Camera for Surveys F606W, F814W photometry of six globular clusters (GCs) are presented. The six GCs form two loose groupings in Galactocentric distance (RGC): IC 4499, NGC6426, and Ruprecht 106 at ∼15-20kpc and NGC7006, Palomar 15, and Pyxis at ∼40kpc. The CMDs allow the ages to be estimated from the main-sequence turnoff in every case. In addition, the age of Palomar 5 (RGC ∼ 18kpc) is estimated using archival HST Wide Field Planetary Camera2V, I photometry. The age analysis reveals the following: IC 4499, Ruprecht 106, and Pyxis are 1-2Gyr younger than inner halo GCs with similar metallicities; NGC7006 and Palomar 5 are marginally younger than their inner halo counterparts; NGC6426 and Palomar 15, the two most metal-poor GCs in the sample, are coeval with all the other metal-poor GCs within the uncertainties. Combined with our previous efforts, the current sample provides strong evidence that the Galactic GC age-metallicity relation consists of two distinct branches. One suggests a rapid chemical enrichment in the inner Galaxy while the other suggests prolonged GC formation in the outer halo. The latter is consistent with the outer halo GCs forming in dwarf galaxies and later being accreted by the Milky Way